A heat exchanger such as a condenser and feedwater heater for a main turbine of a nuclear power plant has many heat exchanger tubes, which are small caliber tubes, installed therein. These heat exchanger tubes are replaced and disposed of when the end of their life comes. However, according to a disposal method of shredding all applicable heat exchanger tubes and filling a drum with shredded heat exchanger tubes, a large number of drums will arise and the disposal cost needed to store and bury these drums will be vast. Thus, it is necessary to reduce the quantity of drums to be disposed of by decontaminating heat exchanger tubes.
For this purpose, the reduction in quantity of drums to be disposed of has been achieved by the method described below. That is, after a heat exchanger tube being cut to fixed lengths, cut heat exchanger tubes are half-cut in the longitudinal direction and radioactive substances adhering to inner and outer surfaces are decontaminated by blasting or the like. Next, a decontamination result is checked by a survey meter and heat exchanger tubes whose radioactive contamination is not confirmed are not disposed of after being packed in a drum and only heat exchanger tubes whose radioactive contamination is confirmed are disposed of by being packed in a drum.
However, according to the method of half-cutting heat exchanger tubes in the longitudinal direction and half-cut heat exchanger tubes are decontaminated and then making measurements by a survey meter, it is necessary to half-cut a vast amount of heat exchanger tubes, posing problems that the number of processes increases, time and labor are needed, and further energy such as power is needed and installation costs of facilities increase.
Patent Document 1 discloses a radiation measuring device allowing radiation measurements of inside-tube-wall surface to be made easily without half-cutting a heat exchanger tube. That is, a radiation detector configured by mounting a scintillator unit on a rod-like transparent light guide unit extended from a photoelectric conversion unit and attaching a light-shielding portion that allows radiation to pass through, but blocks light to an outer surface thereof is disclosed.
The radiation detector disclosed by Patent Literature 1 enables radiation measurements of inner-wall surface of a long tube so that monitoring of radioactive contamination of the inner-wall of a tube can be performed efficiently. However, when such a long radiation detector is inserted into a tube, a radioactive substance may adhere to the detector through contact between the tube and the detector, which makes it necessary to decontaminate the detector each time monitoring is performed or pay close attention so that the detector is not damaged by contact with the tube, other tools, or other members. Further, even if close attention is paid, the detector may be damaged by contact with the tube or other members.